Two Buttes, CO., Mantle Evolution and Potassic Magmas

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The genesis of potassic igneous rocks and the evolution of the subcontinental mantle will be investigated by studying the potassic, mafic rocks at Two Buttes, Colorado, the easternmost Cenozoic igneous intrusives in the western U.S. These rocks are at an extreme geographic position, have unusual compositions, and appear to be above a thick continental root. Field studies will be completed, ages will be determined, and petrographic, electron probe, major and trace element, and isotopic analyses will be made. Major and trace element and Nd and Sr isotope abundances will be compared with those of rocks in contrasting tectonic settings to test theories for magma genesis, for metasomatism of continental lithosphere, and for relationships between melt generation and subduction. Pyroxene textures resemble those ascribed to magma mixing, and isotopic as well as chemical studies will be used to investigate the importance of the process in producing the diverse rock types at Two Buttes and elsewhere.